The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Generally, an automatic transmission achieving more speed stages has been developed to enhance fuel efficiency and provide better drivability.
Such an automatic transmission achieving more speed stages allows improvement to power performance and driving efficiency when downsizing an engine. Providing high efficiency multiple-speeds transmissions with smooth step ratios that can be used as an index closely related to drivability such as acceleration before and after a shift increases secure competitiveness of an automatic transmission in the automotive field.
In the automatic transmission, however, as the number of speed stages is increased, the number of internal components is typically increased. This negatively affects mountability, cost, weight and transmission efficiency.
Accordingly, development of a planetary gear train that improves efficiency with a small number of components may be provided in order to increase a fuel efficiency enhancement effect through the multiple-speeds.
Recently, an eight-speed automatic transmission has been introduced and a planetary gear train for an automatic transmission enabling more shift-stages has been developed.
However, since a conventional eight-speed automatic transmission has gear ratio span of 6.5-7.5 (gear ratio span is an important factor for securing linearity of step ratios), improvement of power performance and fuel efficiency may be minimal.
In addition, if an eight-speed automatic transmission has gear ratio span larger than 9.0, we have discovered that it is difficult to secure linearity of step ratios. Therefore, driving efficiency of an engine and drivability of a vehicle may be deteriorated.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the present disclosure, and therefore it may contain information that does not form the prior art that is already known to a person of ordinary skill in the art.